The present invention relates generally to PIN diode switching circuits and more particularly to a PIN diode switching circuit having a biasing arrangement providing minimum energy (highest Q).
A typical application for a high-Q PIN diode switching circuit is to add tuning capacitance to a high-Q resonator. The resonator could be used as the frequency controlling element in an oscillator or a filter. An example of one type of PIN diode switching circuit is described in U.S. Pat. No. 4,649,354 for a switchable multi-frequency dielectric resonator oscillator. The oscillator includes a transistor that is operable for oscillation at a frequency determined by a feedback resonator connected to its control terminal. PIN diodes, acting as switching means, selectively connect any one of a plurality of feedback resonators to the control terminal of the transistor. The gate terminal of the transistor is connected to ground through an inductor and to the common anode terminals of the PIN diodes of the switching means. The diode switching means includes diodes that have their cathodes respectively connected to microstrip lines, which extend past the adjacently positioned dielectric resonators. The ends of the microstrip lines opposite the diodes are terminated to ground through resistors. An additional diode has its cathode terminated to ground through a resistor. Biasing terminals are connected to the cathodes of the PIN diodes via the respective microstrip lines or the resistor for the application of bias voltages to operate the diode switch.
In operation, one of the diodes in the diode switching means is forward biased while the other three diodes are reversed biased by application of appropriate voltages at the biasing terminals. Such biasing electrically connects one of the three microstrip lines or the resistor to the gate of the terminal of the transistor, while isolating the other three elements. The transistor will oscillate at the frequency determined by the resonant frequency of the dielectric resonator coupled to the gate terminal of the transistor through the diode switching means. To change the frequency at which the transistor is oscillating at, the forward biased diode in the switching mean is reversed biased by the application of the appropriate voltage at its biasing terminal, and a previously reversed biased diode is forward biased by the application of a forward bias voltage at its biasing terminal. To stop the transistor from oscillating, the diodes coupled through the microstrip lines to the biasing terminals are reversed biased and the biasing terminal coupled to the resistor is forwarded biased.
Typical PIN diode switching circuits used for adding tuning capacitance to high Q resonators generally causes problems for such circuits. The problems manifest themselves as stray capacitive reactance which limits the range of switchable resonator reactance, and stray resistance and conductance paths which cause energy loss in the resonator.
What is needed is a PIN diode switching circuit that minimizes energy losses through the switch in both the ON and OFF states of the switch.